M2-T07: build auth UI (login, session bootstrap, forced password change, logout)

- real Mantine login form -> POST /api/auth/login; 401 inline error; redirect when already authed
- ProtectedRoute: loading state, preserves intended destination, gates force_password_change
- ChangePasswordPage forced-change gate -> POST /api/auth/password
- logout control in AppLayout nav -> POST /api/auth/logout
- typed client only; vitest tests for the login flow

frontend/src/test-setup.ts

@@ -1,5 +1,38 @@
 /**
  * Vitest global setup file.
  * Imports @testing-library/jest-dom to extend vitest matchers with DOM assertions.
+ *
+ * Also polyfills browser APIs that jsdom does not implement but Mantine needs:
+ * - window.matchMedia (Mantine uses it for color-scheme detection)
+ * - ResizeObserver (Mantine uses it for responsive components)
  */
 import '@testing-library/jest-dom'
+
+// ---------------------------------------------------------------------------
+// window.matchMedia polyfill (jsdom does not implement this)
+// ---------------------------------------------------------------------------
+Object.defineProperty(window, 'matchMedia', {
+  writable: true,
+  value: (query: string) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: () => {},
+    removeListener: () => {},
+    addEventListener: () => {},
+    removeEventListener: () => {},
+    dispatchEvent: () => false,
+  }),
+})
+
+// ---------------------------------------------------------------------------
+// ResizeObserver polyfill (jsdom does not implement this)
+// ---------------------------------------------------------------------------
+if (typeof ResizeObserver === 'undefined') {
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  ;(globalThis as any).ResizeObserver = class ResizeObserver {
+    observe() {}
+    unobserve() {}
+    disconnect() {}
+  }
+}